ButtonBar, FlexibleHeader, HeaderStackView, and NavigationBar are all implementation details of AppBar. While these components can be used on their own, in practice we expect most typical usage to rely on AppBar. FlexibleHeader is a bit blurry in that there is often value in relying on it solely, but because it is not a true concept in the spec and its behavioral demos largely overlap with AppBar's, it is now a dragons demo.
Pivotal story: https://www.pivotaltracker.com/story/show/156982162
The Floating Action Button should have default hitAreaInsets so that it
adheres the minimum touch target in the Material Design guidelines.
This commit introduces a set of default hitAreaInsets for the different
FAB styles as well as a warning if a button's touch target is smaller
than the 48-point minimum.
Closes#223